Evolution Of Overpressure And Its Influence On Enrichment Of Tight Oil Of Lucaogou Formation In Jimsar Depression

There commonly exists overpressure in Junggar Basin,which has important influence on hydrocarbon formation.So it is necessary to study the hydrocarbon accumulation mechanism and enrichment regularity,which is of great significance to petroleum exploration and resource prediction in the overpressure environment.To elucidate the association of overpressure with enrichment of tight oil,this study analyzed the present-day pressure and the forming mechanism of overpressure as well as reconstructed the paleopressure evolution of Lucaogou formation in Jimusar depression by using geological,geophysical,geochemical data and basin modeling.The research of overpressure system of Jimusar depression showed that the present pressure could be vertically categorized into six zones:normal pressure zone,shallow overpressure zone,shallow transitional pressure zone,intermediate overpressure zone,deep transitional pressure zone and deep overpressure zone.The pressure coefficient and excess pressure of each overpressure zone presented increasing trend.Lucaogou formation locates in deep overpressure zone;its excess pressure has the characteristic of different distribution and can be divided into two pressure structure:sandwich-type and multilayer sandwich-type.The former mainly distributed in the eastern slope zone where the excess pressure is low and the latter mainly distributed in depression zone which has great excess pressure.Jimusar depression experienced multi-stage rapid subsidence period;besides source rock of Lucaogou formation began generating and expulsing hydrocarbon at the end of Triassic,so uncompaction and hydrocarbon generation were the main causes of overpressure.In addition,using the software BasinMod^TM modelled the 1D and 2D formation pressure of Lucaogou formation,and the modeling results are identical with the calculation results.The paleopressure modeling results showed that paleopressure evolution of Lucaogou formation could be divided into three and half cycles of accumulation and expulsion.Based on the achievements of overpressure system of Lucaogou formation,the study on the association of overpressure with enrichment of tight oil was carried out,including the influence on thermal evolution,reservior and distribution of tight oil.The results revealed that overpressure suppress the thermal evolution and hydrocarbon-generation of source rock to some extent,and the degree of inhibition presents linear positive correlation to the overpressure.Overpressure has also improved the porosity of reservoir,reservoir spaces and migration paths of tight oil.Overpressure is the main migration power,which controls the whole distribution of tight oil in the plan and the difference of excess pressure between source and reservoir controls the different distribution of tight oil on profile.